MW3 is now backwards compatible on the Xbox One and joins COD 1, 2, COD 4. WAW, BO and BO2 on the platform. That just leaves MW2, please let this franchise die already, for the collection to be completed. A lot of people play the old games particularly the original Black Ops which had six-figures comfortably for a long time - it would be very interesting to see how these compare to the latest games if Activision didn't censor the player counts due to the low populations.
These old games are streets ahead of COD today and that's not from a nostalgia standpoint either. MW3 wasn't the best for me but I'd happily play that over any of this generation's COD games.